It originally had a 21 in the front but it has the optional factory 19'' wheel now. Do these flex a lot? Will a fork brace help?

Yep they will flex like spaghetti in every conceivable direction, however, even a steel mudguard will go a long way to mitigate it, and you have lugs available on the fork lowers to use. The more you can make the front suspension system work as a solid unit will pay dividends.If one side moves up even a millimetre ahead of the other, by the time that is translated to the contact patch of a 19" tyre, you are looking at masses of misalignment and gyroscopic tomfoolery going on. It's easy enough to weld one up comprising of two hoops, one in front and one behind and bolted to the fork lugs if you can't get a funky machined clamp.

Yep they will flex like spaghetti in every conceivable direction, however, even a steel mudguard will go a long way to mitigate it, and you have lugs available on the fork lowers to use. The more you can make the front suspension system work as a solid unit will pay dividends.If one side moves up even a millimetre ahead of the other, by the time that is translated to the contact patch of a 19" tyre, you are looking at masses of misalignment and gyroscopic tomfoolery going on. It's easy enough to weld one up comprising of two hoops, one in front and one behind and bolted to the fork lugs if you can't get a funky machined clamp.